The present invention pertains to a device for monitoring the health status of a patient and, more particularly, for isolating such an apparatus from external disturbances.
The present invention is an improvement upon finger-ring sensors such as those described in U.S. Pat. No. 5,964,701, issued Oct. 12, 1999, which patent is incorporated herein by reference. Finger-ring sensors may be employed, for example, for monitoring such physiological parameters as blood flow, blood constituent concentration, and pulse rate, all of which may be measured by other means that are more cumbersome, and thus less comfortable, to a patient.
Problems that arise in implementing a sensor that may be worn on a finger include issue of eliminating signal artifacts due to motion of the finger and of not interfering with blood circulation within the finger.
Devices have been advanced that employ a cuff for measuring the arterial pressure in a finger and thus provide ambulatory blood pressure monitoring. Such devices are described, for example, in Yamakoshi, et al., Long-Term Ambulatory Monitoring of Indirect Arterial Blood Pressure Using a Volume-Oscillometric Method, 23 Med. and Biol. Eng. and Comput., (1985), pp. 459-465 and in U.S. Pat. No. 4,539,997 (Wesseling et al.), which are incorporated herein by reference. In these devices, a cuff, encircling the finger, is inflated and pressure is applied uniformly around the finger, thereby interfering with the natural blood perfusion through the vasculature of the finger.
Another feature of some finger ring physiological monitoring systems is noise-cancellation, as taught, for example, in U.S. Pat. No. 6,263,222 (Diab et al.). This patent is also incorporated herein by reference. Diab teaches providing more than one source of light, with the different sources emitting at different wavelengths, for illuminating a single detector along different paths through the skin, and deriving two signals, one of which may be used to remove motion-induced artifacts from the other signal.
In addition to enhancing signal-to-noise in a photoplethysmograph without impeding blood flow through the finger, it would desirable to derive a separate signal that may be used for motion monitoring.